I've encountered below error (vCenter unable to synchronize and all guest VMs become orphaned) yesterday. All VMs are working fine that time.
Login to each host and checked but each host are OK and all guest VMs are running well.

The error is appear on vCenter only. Checked from vSphere Client and Browser was appeared same error.

I try to PING each host IP and Name from vCenter Server.
Just got reply from IP and no reply from PING with name.
Yes, I realized it is DNS issue. vCenter unable to resolve each host's name.

I tried to flush vCenter Server DNS cache by using "ipconfig /flushdns" command via Command Prompt and restart the server once task done.

I login to vCenter after it restarted and noticed that vCenter able to sync with all host and all VMs also back to normal.

This kind of error can be happen in many situation.
but I just share how to resolve because of for other people who encounter same situation like me will not waste their time by reading this post.